The present invention relates to a sewing machine which includes a sewing-machine cutting device comprising a movable upper knife and a stationary bottom knife which cooperates therewith.
The prior art includes a number of sewing-material cutting devices of the general type mentioned above, in which a movable upper knife cooperates with a fixed bottom knife for cutting sewing material. All the known solutions have the feature in common that they have an upper knife of relatively complicated form; see, e.g., Federal Republic of Germany Utility Model 66 04 108, especially FIG. 21.
There is also known an upper knife having a carbide metal tip for edge trimming devices on sewing machines, see Federal Republic of Germany Utility Model 18 89 277, this knife also having a complicated structure.
As a result of these complicated prior art designs, the upper knives known up to the present time have all had the disadvantage of being expensive parts which are subject to wear. Accordingly, it has been the practice for decades to sharpen by regrinding dulled cutting edges which are no longer suitable for use. The regrinding requires a special knife-grinding apparatus which must be equipped with cutting stands for all or most of the different possible cutting angles. Since the regrinding must be carried out by skilled personnel, it is wage-intensive. Another disadvantage is that the reground upper knife must also be installed and adjusted for proper cutting by skilled personnel, in view of the displacement of the cutting edge which takes place by virtue of the regrinding.